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ELODEA LAB
Adapted from Dan Flerlage, CIBT (08)
Background Concepts:
Plants can carry out both photosynthesis and respiration simultaneously. During photosynthesis, plants are using
the energy of the sun to build molecules which effectively store this energy (glucose). Chemically, the
photosynthetic reaction looks like this:
During respiration, plants are using this stored energy (glucose), to fuel their metabolic processes. Chemically, the
respiratory process looks like this:
Remember that plants can carry out respiration all the time!
Among other things, the converted energy from respiration is used to synthesize molecules, move materials around
within the organism, grow (create new cells) and reproduce. Notice that in photosynthesis, CO2 (carbon dioxide)
is being used up as it is “fixed” into glucose molecules. During respiration the opposite is true. As the plant
releases the energy stored in glucose by breaking it down, CO2 is being given off into the surrounding water or
atmosphere. The relationship between these two processes is special in that it allows plants to recycle some of their
by-products. (While CO2 is being given off during respiration, it can be re-utilized during photosynthesis.)
In this lab, you will try to demonstrate the net change in carbon dioxide when the common fresh water plant Elodea
is placed under different conditions. You will be using a pH indicator, bromothymol blue, as a means of
determining the presence or absence of CO2.
A solution of bromothymol blue changes color when CO2 is introduced. It changes color due to a change in pH.
When CO2 is dissolved in water, it forms carbonic acid. This lowers the pH of the solution and causes the
bromothymol blue to change its appearance.
Purpose:
Your lab group is asked to design, execute, and analyze an experiment that tries to accomplish the following
two tasks:
Task A. Demonstrate that environmental CO2 is used during photosynthesis in Elodea.
Task B. Demonstrate that there is a net production of CO2 when Elodea respires in the absence of
photosynthesis.
Background Questions:
1. Why must plants carry out both photosynthesis and cellular respiration?
2. What similarities exist between the reactions for photosynthesis and respiration? What differences do
you see?
3. Why do many students think plants only carry out photosynthesis and animals only carry out
respiration?
4. Explain how plants recycle some of their CO2.
5. Describe what you will be doing in this lab.
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Getting Started:
Your group should obtain a solution of bromothymol blue. This solution changes color when CO2 is introduced.
Bromothymol blue changes color due to a change in pH. When CO2 is dissolved in water, it forms carbonic acid.
This lowers the pH of the solution (makes it more acidic) and causes the bromothymol blue to change its
appearance.
Remember that you are a living generator of CO2. Use a straw to gently exhale into a 250 ml flask containing 20 ml
of bromothymol blue. Continue exhaling into bromothymol blue solution for about one minute.
CAUTION: Be careful not to swallow any bromothymol or splash it in your face. It is toxic!
Record your observations. Do not discard this solution.
When CO2 is removed from bromothymol blue solution, the solution turns back to its original color (blue). To
confirm this, label with your names or the group number the 250 ml flask into which you have just exhaled CO2.
Set it aside for 24 hours with the top uncovered. The CO2 in solution will eventually achieve equilibrium with
atmospheric CO2. The air around us contains relatively little CO2. Therefore most of the CO2 molecules bubbled
into the solution should leave.
Your group will use sprigs (pieces) of Elodea, a water plant. The bromothymol blue will not interfere with
respiration or photosynthesis in Elodea. You have test tubes and corks at your disposal, as well as other materials,
including aluminum foil (which is an excellent way to provide a plant with a totally dark environment). You may use
as many test tubes as are needed for you to design a controlled experiment.
Getting Started Questions:
6. What happened when you exhaled into the bromothymol solution? Why?
7. What will happen to the bromothymol solution after sitting uncovered for 24 hours? Why?
8. What is Elodea? Why is it a good plant to use with bromothymol in this lab?
Procedure:
Day 1: Now that you are familiar with the behavior of bromothymol in relation to dissolved CO2, you are ready to
use it in designing and executing your lab. Go back and read the original “Purpose” on Page 2. The Elodea sprigs
can be cut to fit into the test tube provided. This allows you to give the Elodea a variety of controlled conditions.
The test tubes may be filled with solutions containing abundant CO2 or very little. (Where can you get some CO2
gas?) You may choose to include Elodea in some but not all test tubes. The aluminum foil can be used to control
light. This allows you to set up different test tubes for comparison. Remember to include controls. You may
need more than one test tube for your controls. You may use corks or Parafilm to seal off each of your tubes.
Discuss within your group what design might best accomplish the tasks described under “Purpose.” Once your
group settles on a design, fill out the Experimental Design sheet and have your teacher approve it. After your
design is approved by your teacher, use the supplies available to set up your experiment.
NOTE: To determine the amount of bromothymol solution that should be used in each of your test tubes pour a
filled test tube into the graduate cylinder. Fill a 500 ml flask with all of the bromothymol blue that you will need to
turn yellow. Then carefully blow CO2 into the flask until your solution turns yellow. This will ensure that an equal
amount of CO2 is in each test tube. As you set up your experiment, label the test tubes. On your Data Table Design
Sheet design a data table to record the contents and appearance of each tube. Remember to include space to record
any changes you may observe on Day 2. Your teacher must approve your Data Table Design. Start by recording the
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original appearance of the tubes. Use a label or tag to identify your test tube rack. Your test tube rack needs to be
exposed to a light source wherever your teacher indicates.
Day 2: The test tubes of your experiment have now had approximately 24 hours to carry on respiration and perhaps
photosynthesis. The original CO2 concentration of the test tubes has been altered by these two processes. Today
you will make observations of your test tubes, record changes, and interpret the data you have collected. Fill in the
Data Table with your 24 hour observations and your explanations for the color changes.
Experimental Design:
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What is the TITLE of your experiment?
Restate the two tasks (Task A and Task B from page 1) of your experiment.
What is your HYPOTHESIS?
What is the INDEPENDENT VARIABLE (the condition you change in the experiment) of the
experiment?
What is the DEPENDENT VARIABLE (the condition that you measure in the experiment)?
How will you measure the DEPENDENT VARIABLE?
What is the CONTROL (the condition that you do NOT change in the experiment)?
What MATERIALS are you going to use?
Describe the PROCEDURE of your lab.
Draw the PREDICTED RESULTS DATA TABLE below. Make sure to include the CONTROLS.
Predicted Results - Data Table
Test Tube
Light or Dark?
Plant and/or CO2 added?
Color of BTB at start
Color of BTB after 24 hrs.
Explanation for BTB color
change
A
B
C
D
E
F
G
H
Results/Conclusion Questions:
19. Draw the ACTUAL RESULTS DATA TABLE in the same format as the predicted results table. In
what ways were your predictions correct or incorrect?
20. To provide evidence for Tasks A and/or B you must compare 2 test tubes. To do this pick two test
tubes and indicate how they help to provide evidence to support Task A.
a. Are there other tubes that when compared to each other provide evidence to support Task A?
b. Pick two test tubes and indicate how they help to provide evidence to support Task B.
c. Are there other tubes that when compared to each other provide evidence to support Task B?
21. If your lab results and analysis did not allow you to prove both Task A and Task B, then you need to
redesign a follow-up experiment which would provide the missing data. Explain and illustrate this
follow-up experiment in your report, and explain how it could provide additional information.
22. Write the equation for photosynthesis:
23. Write the equation for respiration:
24. Which process(es) occurs only in the presence of light?
25. Which process(es) occurs both in light and dark?
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